A flirtation with a Finals win followed by another with one of the greatest of his generation has taught the Celtics a lesson on time. Time is valuable. You’ve heard that expression a million times. Time is money. Time waits for no man. These Celtics know all about time. Boston was five minutes away from a 3-1 lead in the Finals. Tick, tock. Precious seconds felt like an eternity whenever Steph Curry had the ball in that perilous fourth quarter. Tick, tock. TD Garden begged for time to somehow speed up. Tick, tock. But the defending champs somehow found a…